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     NAME
          signal-safety - async-signal-safe functions

     DESCRIPTION
          An async-signal-safe function is one that can be safely
          called from within a signal handler.  Many functions are not
          async-signal-safe.  In particular, nonreentrant functions
          are generally unsafe to call from a signal handler.

          The kinds of issues that render a function unsafe can be
          quickly understood when one considers the implementation of
          the stdio library, all of whose functions are not async-
          signal-safe.

          When performing buffered I/O on a file, the stdio functions
          must maintain a statically allocated data buffer along with
          associated counters and indexes (or pointers) that record
          the amount of data and the current position in the buffer.
          Suppose that the main program is in the middle of a call to
          a stdio function such as printf(3) where the buffer and
          associated variables have been partially updated.  If, at
          that moment, the program is interrupted by a signal handler
          that also calls printf(3), then the second call to printf(3)
          will operate on inconsistent data, with unpredictable
          results.

          To avoid problems with unsafe functions, there are two pos-
          sible choices:

          1. Ensure that (a) the signal handler calls only async-
             signal-safe functions, and (b) the signal handler itself
             is reentrant with respect to global variables in the main
             program.

          2. Block signal delivery in the main program when calling
             functions that are unsafe or operating on global data
             that is also accessed by the signal handler.

          Generally, the second choice is difficult in programs of any
          complexity, so the first choice is taken.

          POSIX.1 specifies a set of functions that an implementation
          must make async-signal-safe.  (An implementation may provide
          safe implementations of additional functions, but this is
          not required by the standard and other implementations may
          not provide the same guarantees.)

          In general, a function is async-signal-safe either because
          it is reentrant or because it is atomic with respect to sig-
          nals (i.e., its execution can't be interrupted by a signal

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          handler).

          The set of functions required to be async-signal-safe by
          POSIX.1 is shown in the following table.  The functions not
          otherwise noted were required to be async-signal-safe in
          POSIX.1-2001; the table details changes in the subsequent
          standards.

          lb lb l l.  Function  Notes abort(3)  Added in POSIX.1-2001
          TC1 accept(2) access(2) aio_error(3) aio_return(3)
          aio_suspend(3) See notes below alarm(2) bind(2)
          cfgetispeed(3) cfgetospeed(3) cfsetispeed(3) cfsetospeed(3)
          chdir(2) chmod(2) chown(2) clock_gettime(2) close(2) con-
          nect(2) creat(2) dup(2) dup2(2) execl(3)  Added in POSIX.1-
          2008; see notes below execle(3) See notes below
          execv(3)  Added in POSIX.1-2008 execve(2) _exit(2) _Exit(2)
          faccessat(2)   Added in POSIX.1-2008 fchdir(2) Added in
          POSIX.1-2008 TC1 fchmod(2) fchmodat(2)    Added in POSIX.1-
          2008 fchown(2) fchownat(2)    Added in POSIX.1-2008 fcntl(2)
          fdatasync(2) fexecve(3)     Added in POSIX.1-2008
          ffs(3)    Added in POSIX.1-2008 TC2 fork(2)   See notes
          below fstat(2) fstatat(2)     Added in POSIX.1-2008 fsync(2)
          ftruncate(2) futimens(3)    Added in POSIX.1-2008 getegid(2)
          geteuid(2) getgid(2) getgroups(2) getpeername(2) getpgrp(2)
          getpid(2) getppid(2) getsockname(2) getsockopt(2) getuid(2)
          htonl(3)  Added in POSIX.1-2008 TC2 htons(3)  Added in
          POSIX.1-2008 TC2 kill(2) link(2) linkat(2) Added in
          POSIX.1-2008 listen(2) longjmp(3)     Added in POSIX.1-2008
          TC2; see notes below lseek(2) lstat(2) memccpy(3)     Added
          in POSIX.1-2008 TC2 memchr(3) Added in POSIX.1-2008 TC2
          memcmp(3) Added in POSIX.1-2008 TC2 memcpy(3) Added in
          POSIX.1-2008 TC2 memmove(3)     Added in POSIX.1-2008 TC2
          memset(3) Added in POSIX.1-2008 TC2 mkdir(2)
          mkdirat(2)     Added in POSIX.1-2008 mkfifo(3)
          mkfifoat(3)    Added in POSIX.1-2008 mknod(2)  Added in
          POSIX.1-2008 mknodat(2)     Added in POSIX.1-2008
          ntohl(3)  Added in POSIX.1-2008 TC2 ntohs(3)  Added in
          POSIX.1-2008 TC2 open(2) openat(2) Added in POSIX.1-2008
          pause(2) pipe(2) poll(2) posix_trace_event(3) pselect(2)
          pthread_kill(3)     Added in POSIX.1-2008 TC1
          pthread_self(3)     Added in POSIX.1-2008 TC1
          pthread_sigmask(3)  Added in POSIX.1-2008 TC1 raise(3)
          read(2) readlink(2) readlinkat(2)  Added in POSIX.1-2008
          recv(2) recvfrom(2) recvmsg(2) rename(2)
          renameat(2)    Added in POSIX.1-2008 rmdir(2) select(2)
          sem_post(3) send(2) sendmsg(2) sendto(2) setgid(2)
          setpgid(2) setsid(2) setsockopt(2) setuid(2) shutdown(2)
          sigaction(2) sigaddset(3) sigdelset(3) sigemptyset(3) sig-
          fillset(3) sigismember(3) siglongjmp(3)  Added in POSIX.1-
          2008 TC2; see notes below signal(2) sigpause(3) sigpend-
          ing(2) sigprocmask(2) sigqueue(2) sigset(3) sigsuspend(2)
          sleep(3) sockatmark(3)  Added in POSIX.1-2001 TC2 socket(2)

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          socketpair(2) stat(2) stpcpy(3) Added in POSIX.1-2008 TC2
          stpncpy(3)     Added in POSIX.1-2008 TC2 strcat(3) Added in
          POSIX.1-2008 TC2 strchr(3) Added in POSIX.1-2008 TC2
          strcmp(3) Added in POSIX.1-2008 TC2 strcpy(3) Added in
          POSIX.1-2008 TC2 strcspn(3)     Added in POSIX.1-2008 TC2
          strlen(3) Added in POSIX.1-2008 TC2 strncat(3)     Added in
          POSIX.1-2008 TC2 strncmp(3)     Added in POSIX.1-2008 TC2
          strncpy(3)     Added in POSIX.1-2008 TC2
          strnlen(3)     Added in POSIX.1-2008 TC2
          strpbrk(3)     Added in POSIX.1-2008 TC2
          strrchr(3)     Added in POSIX.1-2008 TC2 strspn(3) Added in
          POSIX.1-2008 TC2 strstr(3) Added in POSIX.1-2008 TC2
          strtok_r(3)    Added in POSIX.1-2008 TC2 symlink(2)
          symlinkat(2)   Added in POSIX.1-2008 tcdrain(3) tcflow(3)
          tcflush(3) tcgetattr(3) tcgetpgrp(3) tcsendbreak(3) tcse-
          tattr(3) tcsetpgrp(3) time(2) timer_getoverrun(2)
          timer_gettime(2) timer_settime(2) times(2) umask(2) uname(2)
          unlink(2) unlinkat(2)    Added in POSIX.1-2008 utime(2)
          utimensat(2)   Added in POSIX.1-2008 utimes(2) Added in
          POSIX.1-2008 wait(2) waitpid(2) wcpcpy(3) Added in POSIX.1-
          2008 TC2 wcpncpy(3)     Added in POSIX.1-2008 TC2
          wcscat(3) Added in POSIX.1-2008 TC2 wcschr(3) Added in
          POSIX.1-2008 TC2 wcscmp(3) Added in POSIX.1-2008 TC2
          wcscpy(3) Added in POSIX.1-2008 TC2 wcscspn(3)     Added in
          POSIX.1-2008 TC2 wcslen(3) Added in POSIX.1-2008 TC2
          wcsncat(3)     Added in POSIX.1-2008 TC2
          wcsncmp(3)     Added in POSIX.1-2008 TC2
          wcsncpy(3)     Added in POSIX.1-2008 TC2
          wcsnlen(3)     Added in POSIX.1-2008 TC2
          wcspbrk(3)     Added in POSIX.1-2008 TC2
          wcsrchr(3)     Added in POSIX.1-2008 TC2 wcsspn(3) Added in
          POSIX.1-2008 TC2 wcsstr(3) Added in POSIX.1-2008 TC2
          wcstok(3) Added in POSIX.1-2008 TC2 wmemchr(3)     Added in
          POSIX.1-2008 TC2 wmemcmp(3)     Added in POSIX.1-2008 TC2
          wmemcpy(3)     Added in POSIX.1-2008 TC2
          wmemmove(3)    Added in POSIX.1-2008 TC2
          wmemset(3)     Added in POSIX.1-2008 TC2 write(2)

          Notes:

          *  POSIX.1-2001 and POSIX.1-2001 TC2 required the functions
             fpathconf(3), pathconf(3), and sysconf(3) to be async-
             signal-safe, but this requirement was removed in
             POSIX.1-2008.

          *  If a signal handler interrupts the execution of an unsafe
             function, and the handler terminates via a call to
             longjmp(3) or siglongjmp(3) and the program subsequently
             calls an unsafe function, then the behavior of the pro-
             gram is undefined.

          *  POSIX.1-2001 TC1 clarified that if an application calls

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             fork(2) from a signal handler and any of the fork han-
             dlers registered by pthread_atfork(3) calls a function
             that is not async-signal-safe, the behavior is undefined.
             A future revision of the standard is likely to remove
             fork(2) from the list of async-signal-safe functions.

          *  Asynchronous signal handlers that call functions which
             are cancellation points and nest over regions of deferred
             cancellation may trigger cancellation whose behavior is
             as if asynchronous cancellation had occurred and may
             cause application state to become inconsistent.

        errno
          Fetching and setting the value of errno is async-signal-safe
          provided that the signal handler saves errno on entry and
          restores its value before returning.

        Deviations in the GNU C library
          The following known deviations from the standard occur in
          the GNU C library:

          *  Before glibc 2.24, execl(3) and execle(3) employed
             realloc(3) internally and were consequently not async-
             signal-safe.  This was fixed in glibc 2.24.

          *  The glibc implementation of aio_suspend(3) is not async-
             signal-safe because it uses pthread_mutex_lock(3) inter-
             nally.

     SEE ALSO
          sigaction(2), signal(7), standards(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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